The passage of an electron beam over a metallic grating structure generates radiation that can be used in mm-wave and sub-mm-wave (Thz) spectroscopy. The grating structure is also known as a slow wave structure in which the electromagnetic field travels at a rate slower than the speed of light. The amount of radiation emitted is maximized by passing as high a current through an interaction region over the grating structure as possible. At a typical operating voltage (e.g., 5 kV) the depth of this interaction region is on the order of 10-20 microns high, while the width of the region is some significant fraction of the grating width (e.g., 10 mm or 1 cm), i.e. 1 cm by 20 microns. Thus a very high aspect ratio of 500 to 1 of beam width to beam thickness. Since only the portion of the electron beam passing through this interaction region contributes to the generation of radiation, for maximum efficiency the electron beam should feature roughly the same cross-section as the interaction region, i.e., the beam should be a ribbon beam that is several millimeters wide, with a constant beam height over the grating on the order of several tens of microns. This is in contrast to electron beams presently used for this purpose (interacting with a slow-wave structure), which are typically round and much larger than the interaction region.
In order to maintain a constant beam size over the grating, it is common for the grating to be placed in a magnetic field oriented in the same direction as the beam motion. A schematic of this is illustrated in conventional THz source 100 configuration in FIG. 1A. This field is typically on the order of 0.5 T in magnitude, and can be produced by rare-earth permanent magnets 102. The total beam current 104 emitted by the electron gun 106 is limited by the emission capabilities of the cathode and the size of the emitting area on the cathode. To maximize the amount of current in the electron beam 104 for a given cathode type, one wants to draw current from as large an emitting area as possible. One can then use electrostatic focusing to reduce the thickness of the beam 104 to the desired value over the grating structure 108. The limiting factor of this approach is the magnitude of the Larmor radius (rotations of the electrons in the beam) characteristic of the electrons in the beam as they move through the magnetic field in the grating region. The Larmor radius is RL=mvr/qB, where mvr is the transverse momentum of any given electron, v is for velocity, q is electric charge and B is the magnetic field. So while focusing can be used to reduce the beam size at the grating 108, too much focusing introduces excessive transverse momentum, leading to a large Larmor radius that will actually enlarge the beam size over the grating. This represents a tradeoff that can be optimized through a coordinated design of the electron gun and magnets, as described herein below.
The requirement that any electrostatic focusing introduce minimal transverse momentum to the electrons in the beam constrains the gun lens region to have a long focal length, and hence requires the gun 106 to be positioned a sufficient distance form the grating 108. In principle, the longer the focal length and greater the distance the gun 106 is from the granting 108, the smaller the beam 104 can be made at the grating structure 108. In practice, the displacement is limited by the constraints on the desired size of the device, and by emittance and space-charge considerations.
Thus, there is a need in the art to provide an improved electron gun and magnetic circuit, thereby improving the function and efficiency of an electromagnetic wave radiation source configuration and overcome the disadvantages of the prior art.